【作者】 郭捷；

【导师】 江道灼； 梁一桥；

【作者基本信息】 浙江大学 ， 电气工程， 2013， 博士

【摘要】 电力电子技术的不断发展为建设智能、清洁、高效的现代电力系统提供了强大的支持,在高压直流输电(HVDC)和柔性交流输电(FACTS)领域均取得了广泛的应用。模块化多电平换流器(Modular Multilevel Converter, MMC)作为新一代高压大功率换流装置,当其在HVDC中应用时,具有输电容量大,有功无功可独立控制,交流输出无需复杂的滤波装置,可靠性高,可向无源或弱受端系统输电等诸多优点,被认为是柔性直流输电的代表性技术。本文在前人研究成果的基础上,重点研究MMC在HVDC中应用的关键问题。MMC电路中具有大量电力电子开关和直流电容器,其数学模型呈高阶、离散、非线性的特点,难以建立准确的数学模型进行理论分析。以其物理电路模型为基础,建立MMC的开关模型,通过简化程度不同的三次降阶化简和坐标变换,推导MMC的平均值模型,通过小信号线性化,推导出低阶、连续、线性的MMC的小信号模型。提出的建模方法明确了推导过程中的简化条件,以便于根据不同的分析要求选择对应的简化条件。提出的空间状态数学模型具有直流侧输出电压控制量,使MMC的数学建模更加完整,并为n可控的调制方法提供了理论基础。MMC在HVDC中应用时,换流器主要采用最近电平调制法(Nearest Voltage Level Modulation, NLM)进行阀组级控制。但何时及如何实施其中的选择性子模块投切策略,以及如何避免两次相邻投切操作产生冲突,需要详细研究。定义了NLM法进行子模块电压选择性投切的控制时机,给出了控制时机的产生算法和冲突避免规则。在定义基础上,根据不同类型的控制时机分别提出了相应的子模块选择方法,并对它们进行了对比,给出了控制时机的协调搭配方法。提出的正负极间子模块数n可控的NLM调制方法,可以充分利用MMC中的冗余子模块将其直流侧在一定条件下等效为可控电压源,为MMC直流电流控制提供了实现途径。对提出的NLM法进行了进一步改进,提出了n可以为偶数也可以为奇数的改进型NLM法和适用于带有全H桥子模块的MMC的NLM调制方法,对NLM的应用范围进行了拓展。n可控的NLM调制法可以将MMC的交流侧和直流侧均等效为一个受控电压源,利用这一特性可以进一步丰富MMC的控制手段,以实现更好的系统特性。提出了MMC直流电流直接控制及其参数设计方法,以及当内环采用直流电流控制时外环输出控制器的参数设计方法,并给出了当系统包含MMC直流侧直接电流控制时的控制策略和功率平衡限制条件。电力系统中存在功率单向输送的情况,此时可以采用送端使用二极管整流器,受端采用MMC逆变器的ULMMC-HVDC结构,降低系统复杂度和成本,提高系统可靠性。提出电容储能的ULMMC-HVDC利用MMC中存在的大量子模块,将其与EDLC相结合。弥补了EDLC自放电率较大、耐压较低的缺点,使子模块电压平衡控制容易实现,能量分布存储使可靠性提高且易于维护。提出了能量管理控制器及其参数设计方法,用于限制存储在换流器中的能量在设定的上下限之间。提出电池储能的ULMMC-HVDC,可以使装置整体实现较高的额定电压接入电力系统,且便于电池元件的维护、更换。相较于电容储能,进行长期储能时可以得到更高的存储容量和充放电效率。提出了子模块和子模块组电池荷电状态(State of Charge, SOC)平衡控制及其参数设计方法。大规模MMC电磁暂态仿真模型中存在大量子模块和电力电子元件造成模型阶数过高、仿真效率过低的问题,需要研究提高仿真效率的方法。在MMC仿真电路模型与MMC开关函数数学模型完全等效的前提下,通过改善子模块快速仿真模型和控制系统优化仿真算法等手段,极大地提高了仿真效率,从而使大规模MMC-HVDC的仿真研究变得可能。设计了MMC-HVDC控制系统的优化仿真算法和数据结构,通过两次排序解决子模块选择性投切问题和单个子模块开关频率过高的避免机制。实际MMC装置中存在大量子模块,硬件上需要大量高吞吐能力1O进行测量控制,使控制难度增加；MMC控制系统一般由多控制器协调控制,但这些控制器间需要交互大量实时和非实时信号,难以保证同时可靠传输。研制了由400V AC/±400V DC/3kVA/288-SM的小模型主电路和全比例控制系统组成的MMC验证样机。设计了MMC验证样机的硬件和控制软件结构。研究并设计了子模块测控信号复用电路,在不增加通讯负担的前提下,仅通过增加简单的数字电路,使IO数量与光纤通道数大为减少,降低了装置成本和实现难度。提出了控制系统的双总线结构,通过高速数字1O总线和CAN总线分别传输实时和非实时信号,解决了实控制器间数据交互问题。进行了MMC逆变并网实验,实验结果验证了MMC基本原理和控制策略,证明了MMC验证样机各项性能符合设计指标。更多还原

【Abstract】 The application of power electronic technology constantly plays a significant role in modern power system to make it intelligent, clean, and efficient, including high voltage direct current transmission (HVDC) and flexible alternating current transmission system (FACTS). Modular multilevel converter (MMC) is considered as a next-generation technology of high voltage and high power conversion. MMC is considered as a representative application in flexible-HVDC technology, with the advantages like high capability, decoupled active and reactive power control, high level of power quality with no need of complicated ac filters, high reliability, and ability to weak ac networks or even dead networks. In this dissertation, several key issues in MMC for HVDC application are amply discussed following a literature review.In MMC circuit, there are a large number of semiconductor switches and dc capacitors. These components leads to a high-order, discrete and non-linear mathematical model which is hard to be deduced and analyzed. Based on the physical lumped parameter circuit model, a switching model is given and simplified to an average model through three steps of simplification and coordinate transformation. Finally, a low-order, continuous and linear model is given through small-signal linearization. The preconditions of the proposed simplification are defined to offer different degrees of simplification according different analysis requires. With dc voltage controlled variable, the proposed state-space model makes MMC mathematic model more complete in control theory, and provides a theoretic foundation for the n-controllable modulation for MMC.In MMC-HVDC application, nearest voltage level modulation (NLM) is widely reported and used for the valve group control. However, selective switching strategy based on submodule (SM) voltage and collision avoidance rules of two adjacent switching operations are still needed to be discussed in detail. In this dissertation, control points of the selective switching strategy are defined. An algorithm and collision avoidance rules for the control points are also given. The algorithm proposes different selecting methods corroding different type of control points, gives the rule of control points design. An n-controllable NLM (n is the number of SM per phase) is proposed to control the dc side of MMC as a controllable voltage source under certain conditions for dc current control, and to make a full use of redundancy SMs from the control point of view. Modified strategies of n-controllable NLM are proposed to expand the application of NLM for odd n or H-bridge SMs.With the n-controllable NLM, dc and ac sides of MMC are controlled as controllable voltage sources which provide a fundamental base for various system control strategies in a higher level. A direct dc current control strategy and its parameter design method are proposed for an inner-loop controller of MMC in this dissertation. As a coordinative outer-loop controller, an output control strategy, a system control strategy and their parameter design methods are proposed with power balance limitation.There are cases of unidirectional power transmission in power system especially for renewable energy source connection and offshore platform power supply. It is proposed to reduce system complexity and cost and to improve system reliability that a unidirectional low-cost MMC-HVDC (ULMMC-HVDC) system with a diode rectifier and a MMC inverter. Distinguished by energy storage elements, two types of energy storable ULMMC-HVDC is proposed and discussed. Parallel connected with electronic double layer capacitors (EDLCs) on dc side, the large number of SMs are fully used to achieve the advantages as good performance of SM voltage balancing control, distributed storage that is flexible and easy to maintain. The proposed system also offsets the self-discharge and the low voltage rating problems brought by EDLC. An energy management control strategy and its parameter design method are given to limit the energy stored in EDLC between upper and lower bound. Parallel connected with battery on dc side of SM, energy storable ULMMC-HVDC achieves higher voltage rating compared with conventional battery energy storage system, and makes damaged battery models easily to be maintained or replaced. Compared with supper capacitor energy storage system, it provides a storage scheme with higher capacity and efficiency for power system. A state of charge (SOC) control strategy and its parameter design method are also given for SMs and SM clusters.Electromagnetic transient simulation model for large-scale MMC has the problems of high-order and low-efficiency, because there is a large number of SM with semiconductor switches and dc capacitors in the circuit. As a completely equivalent model of the switching model of MMC, a proposed simulation model greatly improves the efficiency and provides a feasible way for the simulation through building fast-simulation SM model and optimized control algorithm. The algorithm includes double-sorting operations for selective switching and collision avoidance rules for over frequency of SM switching.One of the main problems in practical MMC is a large number of SMs requests high-throughput digital IOs for control and measurement. The control system of MMC is generally a multi-controller system. However, another problem is to realize an interface for high-speed real-time communication and large amount of non-real time data between the controllers. A MMC prototype, including a400V AC/±400V DC/3kVA/288-SM main circuit and a full-scale control system, is developed and tested. The hardware design and the software structure are given in this dissertation. A control and measurement signals multiplexer for SM is proposed to greatly reduce the cost and difficulty, by reducing the number of fiber channels, using only logical circuits without bringing in any communication burden. A double-bus structure is proposed to transmit real-time and non-real time data separately. The structure solves the problem of data interaction between the controllers. The experimental results verify the fundamental principle of MMC and the proposed control strategies. All performance indexes of the prototype are adherent to the designed specification upon examination.更多还原